The invention relates to a method and an apparatus for surface imaging, wherein the surface is scanned by a primary particle beam and secondary and/or back-scattered particles released at the surface are detected by a detector.
Such methods and apparatus are used, for example, for undistorted surface imaging and critical dimension measurement in particle beam systems and especially in the semi-conductor industry.
In order to guarantee high yield and reliable electrical specifications, integrated circuits have to be manufactured within tight dimensional tolerances. Therefore, each manufacturing step is controlled in a particle beam apparatus, mainly a scanning electron microscope, regarding process quality and critical dimension tolerances. Today""s devices with 0.18 xcexcm design rules already require a critical dimension measurement accuracy in the lower nanometer range.
Since integrated circuit process layers, which have to be inspected, are formed from electrical insulation material (e.g. resist layers), electrical charging during the surface imaging with the particle beam may cause problems regarding beam distortion. Such surface charging may not only generate artifacts in the image contrast but can also result in dimensional distortions of the structures to be measured. This may cause measurement errors in critical dimension measurements of several nanometers, which even may reach several tens of nanometers.
Today""s process inspection and critical dimension measurement equipment uses high performance low voltage secondary electron microscopes. Low voltages are supplied in the range of 0.5-2 keV to minimize charging effects that cause inaccuracy in structure imaging by distorting the image profile of secondary electrons. Charging effects become more serious for insulator targets, since no current flow is allowed within the materials.
FIG. 2 shows a typical yield curve for an insulating material. As the primary electron energy increases, the total yield increases until a maximum is reached and then decreases gradually. Two operating points E1 and E2 exist at which the total yield becomes 1, which means that the number of incoming primary electrons and the number of escaping secondary and backscattered electrons is the same:
iPE=iSE+iBSC
Usually, point E2 is preferably selected as the operating condition to avoid charging accumulation. In practice, however, this compensation effect can only be achieved on average for a major surface area, because the secondary electron yield depends not only on the material but also on the local shape, especially the tilt angle of the pattern to be measured:
SE-yield (tilt angle xcex1)=SE-yield (xcex1=0)/cos xcex1
Consequently, there is a local charging effect which is demonstrated in FIG. 3. The pattern shown in FIG. 3 has a side-wall angle xcex1. After scanning of such a pattern, the following charge distribution may be obtained at a certain primary beam energy: Positive charges are found mostly around the edge corners, while negative charges are distributed mainly on the top surfaces and along the side walls.
This local charging will, of course, influence (deflect) the primary particle beam and will consequently generate image artifacts and measurement errors in critical dimension measurement.
The object of the invention is to improve the method and apparatus for surface imaging by minimizing the influence of the local charging effect.
This object is achieved according to the invention in that there are means provided in the region of the surface to be imaged which are supplied with a variable voltage in order to establish a constant surface potential. The variable voltage is then used for image generation of the surface.
Usually, the detector signal is used for image generation. However, the detector output signal according to the invention is kept constant by controlling the variable voltage. Accordingly, the variable voltage is used for image generation.